If you can clip out the rock around the base that would be the best way to remove it, but if you can't do that you can just use your fingers, or even a credit card, to work the base of it off the rock. Leathers are resilient to being torn or cut but keep in mind that doing it that way will leave pieces of it behind that likely will grow into a new colony./
I recently removed one from my tank that was about 3 feet by 2 feet wide and was attached to many rocks. I pulls some of the rocks off but made sure to flip them over, so any fleshy bit left behind got buried in the sand, otherwise I would have several new colonies sprout up in the tank.